Field of the Disclosure
The present disclosure relates to a method and apparatus for manufacturing foils with a thermoplastic surface comprising high-aspect ratio micro or nanostructures.
Background of the Disclosure
In biotechnological, medical and consumer applications, it is desirable to apply functional structures e.g. nanostructures, to defined areas of articles for use as functional surfaces, altering the properties of the surface relative to that of an unstructured surface. Examples of desirable functions are self-cleaning or super repellent surfaces. A method of producing such articles independently of the overall macro-geometry is desirable, in particular if such articles are mass produced at a relative low price as many of these articles must be disposable or low cost reusable products, e.g. toys or packaging material.
The most commonly used method for making controlled micro or nanostructures in thermoplastic surfaces are variotherm injection molding type processes. By melting a thermoplastic material and injecting it into a heated mold under high pressure, the surface of the mold will be replicated, thereby generating a micro or nanostructures polymeric replica. The most common application of this is CD/DVD/Blu-Ray manufacturing, where a polymeric replica may be made in a few seconds. However, the molding of high aspect ratio structures, where the width is low and the depth is high, is challenging using these types of processes due to the rapid cooling of the melt surface upon injection into the cold mold. One solution to this problem has been to vary the temperature of the mold during the process in a variotherm process where the mold is heated above the solidification temperature during melt injection and subsequently cooled below the solidification temperature in order to make the polymeric part solidify so it can be removed from the mold. This, however, increases the cycle times considerably.
Embossing processes are closely related to the variotherm injection molding types of process, where a solid thermoplastic substrate, typically a foil, is being heated while in contact with a master structure made by conventional lithographic means, the master structure typically consist of a nickel or silicon or silicone (PDMS) shim or stamp. After heating and shaping of the surface topography of the substrate to be the inverse of the master structure, the master and substrate is cooled below the solidification temperature of the substrate, and the substrate may be removed. Typical processing throughputs of these types of processes are. cm2 per heating/cooling cycle which typically takes from 10 s and up to several minutes depending on the apparatus, giving a productivity on the order of 10-100 cm2/s equaling 0.001-0.01 m2/s.
Some reports of high speed replication have been given, but only for low aspect ratio structures, typically decorative or diffractive structures.
For many applications these throughput rates are several orders of magnitude too slow. Applications such as functionalized foils for food packaging, coating of windows, ships or car windshields with self cleaning surfaces all require throughputs on the order of 1 m2/s or higher in order to be economically feasible.
Due to the abovementioned problems with the state-of-the-art, it would be desirable to have a technological solution, where high aspect ratio micro or nanostructures may be formed in foils at low cost at high throughput rates. It would further be advantageous if this solution could provide micro or nanostructures of a high quality and it would be a further advantage if the micro or nanostructured area could cover the whole area of the manufactured foil.